Five years of chlorophyll concentrations provided by the SeaWiFS sensor are analyzed to calculate the interannual and total signal-to-noise ratios in the tropical oceans. The signal-to-noise ratios are used to compare the variance of the monthly concentrations against the variance of unresolved spatio-temporal processes. The results show that at 1¿ ¿ 1¿ resolution, variance associated to unresolved processes is as large as the interannual variance in large areas of the tropical oceans, with the exception of the equatorial Pacific. These large errors make it difficult to extract information from the assimilation of five years of interannual anomalies into biogeochemical models in large areas of the tropical oceans. Improvements obtained by using a 1/2¿ ¿ 1/2¿ grid are limited to regions of strong small-scale variability, such as western boundary currents or coastal upwelling. The results also indicate that five years of SeaWiFS data provide potentially useful information about the annual cycle in most regions of the tropical oceans.